Physical Conditions in Dense Molecular Knots in the Supernova Remnant Cassiopeia A

There are many indications that supernova ejecta are important contributors to the dust budget of galaxies, both locally and in the early Universe. However, the formation of this dust and its protection against the destructive effects of the strong reverse shock processing supernova remnants before they merge with the interstellar medium are not well understood. It is likely that dense ejecta knots play a role. Herschel PACS (plus Spitzer, Akari, and ground-based IR) studies of dense knots in the Cas A supernova remnant have revealed large column densities (4e19 per square cm) of warm (500-1000K) dense (1e5 to 1e6 particles per cc) CO gas. This dense environment is very conducive to dust formation and protection. However, the relationship of the molecular and ionic gas is unclear and the derived large column densities are much larger than shock models predict, indicating the importance of energy conduction by electrons from the surrounding hot plasma into the knot. To further our understanding of these knots, we have been granted SOFIA FIFI-LS observations of [OIII] 52&88 µm and [OI] 63 µm fine-structure lines in three CO-rich knots in Cas A. The observations are ongoing, so I will only present preliminary data.